Electrically conducting films and process for forming the same



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ELEGTRJCALLY Cw'ucrrfmc FILMS mi@ PROG-Ess FOR FORMING ,THE Siu/5:

Filedl Sept. 15. 1950' 3 Sheets-Sheet 1 2100 '2o 4o s aosoo 2o' 4o 'en ab' Soo 2o 4o 5o 8o 70o I WAVELENGTH 1N MILLIMICRONS '-INVENTOR.'

ATTORNEYS,

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3 SheetS-Sheet 2 D. A. LYON ELjEcRICALL-Y GONUUCTIN'; Frm/.S AND PR'ocss FOR FORMING THE SAME:

Flled Sept l5 July 24,1956

Filed Sept. l5, 195C D. A. LYON ELSCTRICALLY 'corswcnes .Uw-5 ,ma FRocss 'FCR Femme THE SAME 3 Sheets-Sheet 3 5MM, wl azz/W LWENTOR:

ATTOMYS.

Von the article by this treatment. The lrn is then rendered temperature.

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* nitedtates ELECI'RICALLY COZDUCTIN G FILMS AND ,PROCESS FORFCRMING THE SAME This invention relates tothe product formed. and 'the 1o process for forming an electrically conducting coating or film 'on the surfaces of solid supporting materials or bodies such as plastics, paper, leather, ceramics, quartz, wood, glass, and crystals used in electronics and optics such vas alkali halides, ammonium and potassium dihy drogen phosphates, lithium fluoride, calcium fluoride, titanium dioxide, and the like. l

The prior art teaches the application of thin lms t the,surfaces of high melting point substances such as ceramics and glass so as to provide `a film which is an electrical conductor. Some of these conducting tlms are highly colored and/or opaque whereas-others are transvparent and essentially colorless and capable oftransrriit" ting a high percentage of visible light. However, the

methods of the prior art for applying these electrically a0 vconducting 'tilrns involve' the use of high temperatures 'n excess of 200 C. and usually in the range of 450 There is, for example, a prior art .method of applying a conductingfilm of a compound of silicon to ceramics in which the ceramic article to be coated is heated to -a temperature of ZOO-400 C. and is then subjected to the vapor of silicon tetrachloride. An iridescent but non conducting tilm of a c ompound of Silicon is formed electrically conducting by again heating the coated article to a temperature of ZOO-400 C. in a reducing atmosphere. I

Another example of the prior art for producing a conducting lnrinciudes spraying an appropriate tin comv 50 non of a film on a material which will withstand temperatures of at least 200 C. and usually temperatures of from 450 to 800 C. A 'most important advantage of'the present inventionl over these prior art methods is that the films produced as described herein may be coated on articles which are at substantially roorn ternperature, i. e., 20 C. to 23 C., or even below room The invention enables heat sensitive materials such as those described above to be coated with an electrically conducting coating as wel as permitting heat resistant ceramics and glasses to'be coated.

Accordingly a principal' object of the present invention is to provide the surface of a solid supporting body an electrically conducting coating orir'n which may be produced if desired at room tempera-ture or below and does not require the h igh temperatures used by the prior art processes.

Another object of this invention is to provi-dc the surace of a solid body with an' 'electrically conducting .raneparent coating or film of cuprous iodide. A funher obect or' this invention is to provide a new process for forming an elegtrically ,conducing coating @tice -on .a heat sensitive solid' body su:h as, for example, av

plastic, wood, leather, or'crystalline material used in 'electronics or optics. Furthermore, if it is desired, the coating may be made transparent.

Another object of the invention is to provide a new process for producing an electrically 'conducting coating cn a ceramic or glass or other high melting point rna terial where it is sometimes undesirable or impractical to utilize other processes which require the use of high temperatures. This film or coating may be transparent if desired.

Another object of this inventioni-s to providea process for forming an electrically conducting coating or film on the surface of an electrically non-conducting heat sensitive transparent material. This coating or lm may be transparent if desired.

Still another object of this invention is to provide an article in the form of a solid body coated with an electrically conducting coating which is .preferably but not necessarily transparent. The solid bodycoated by the films of this invention may be heat sensitive and/or nonconducting, and a preferred form of the article is a heat sensitive transparent body coated with a vconducting transparent til'm.

These Aand other objectsof this invention will become apparent upon consideration of the following description taken in connection with the drawings wherein:

Figure l is a graphical illustration of reflectance and transmittance in per cent for various wavelengths of light of various thickness films of this invention coated on transparent plastic; A

Figure 2 is a'vertical cross-sectional view of apparatus suitable for the tir'st stage of one of the preferred forms of t'ne process of this invention;

Figure 3 is a' vertical cross-sectional view of apparatus suitable for the second and final stage of this form of the process;

Figure 4 is an elevational View partially in cross section showing schematically an electrical or mechanical meter employing a sight window made in accordance with this i invention;

Figure 5 is a cross-sectional stew showin g schematically a gaseousdischarge lamp employing a light opening made in accordance with this invention; and

Figure 6 is a perspective vview of an electrical condenser made in accordance with this' invcntit'm.`

I have discovered that the chemical compound known as cuprous iodide (Cul) may be applied to a surface. which vis at room temperature or even below in accordance with the process of the three exemplary methods described hereinafter to 'produce a coating or lm which is a good conductor of electricity, .ransrnits a high percentage of visible light and has but littleopticnl absorption.

Since the'tltic'snesses of the-se lrns of cuprous iodide arc extremely minute, it is most convenient to .express-film thickness in terms of wavelengths of light. T he arbitrary reference standard chosen is"r'nonocl1 romatic light having a wavelength of 5 50 millimicrons. This value has been selected since it represents the 4posit-ion in the visible spect'rum most sensitive to the human eye. Where reference Vis'made in this specification to the thickness Vof a film,

it is understood to be the optical thickness and not the actual mechanical thickness. The optical thickness is equal t'o the mechanical thickness multiplied by the index.

"Q "cloudy in appearance and as the thickness `increases the films approach translncency. Hence, the effective range 2,755,155 Patent-ed Jury a4, `195e.

offusefnlness `Where transparency is a prerequisite, is lime ized to a rnazirnurn thickness of substantially two wavelengths. 'Films of such minute thickness cxhib'itwell known and understood optical effects connected with the constructive and destructive interference of light. Since the index of retraction' of cuprcus iodide (2.35).;5 much higher than that of thel usual materialson which the filmI is formed, i-t Wili be understood by .those skilled in the art't'hat til-ms whicharc 1,4 wavelengths and odd integer rrmltiplcs thereof (such as 1,54, 5/4, f/4, etc.) in thickness, will exhibit mimurn reitection, and minimum transmission, of light` Ort the other hand, films which are even integer multiplesof 1li wavelength Vin thickness (such as Ei, 4/4, and t, etc.) exhibit minimum reection and maximumtransmission of light. Hence, it follows from these considera- Y I to those skilled in the art, and no extended discussion of the same, therefore, appears necessary. C uprous iodide films which are odd multiples of A wavelength in thickness have a'yellowish'-green.hu e by reflected light, whereas those which are even multiples of 1A wavelength in thickness, appear reddish-purple by 'retleeted light. Other colors than'those mentioned signify film thicknesses which llie between even and odd multiples of V4 wavelength in thickness. These colors provide a convenient and simple method for determining hlm thickness by visual examination. Hence, to produce films of maximum transparency,

etc., wavelengths thick. On the'other hand,

the thickness should be such that a reddish-purple retlec- I tion, the thickness should be suol-1 that ayellowish-green reflection isvobserved.

iiteghas been experimentally determined that the films of this invention are equally effective as conductors whether used under normal atmospheric pressure conditions or under vacuum conditions.

The'clectricai resistance measured z cross opposite sides of a square film made in afcordance with this invention is dependent upon the thickness of the cuprous iodide film and upon itscrystalline structure. The resistance of the best quality hints which show little, if any, clottdiness, is

' lower than ilrns of equal thickness which show cloudif ness. Since' it is virtually impossible to prepare films which are absolutely uniform in crystalline structure, it is diiiieult to state a definite single' value for the resistance per square of any given thickness of hlm. The following' table shows the order of magnitude of the resistance per square as a function of thickness:

.if the `thickness of the film is below 1,4 wavelength, the

resistance values increase much more rapidly than would be suggested by the proportional values given in the table. I have made transparent lms which initially measured as low as 150 ohms per square in resistance,

but it has been impossible to maintain these values over any extended period of time, and I have been unable to duplicate these values consistently, If transparency of the hlm is not required, films greater than two wavelengths k 4thick may he made and ldwer'resistanccs thereby obtained.

Pig-ure l of the drawings' graphically illustrates the cxcecdingiy good 'quality of transmission of coated articles by' this invention. The specimens used for the `determination of reflection and transmission in per cent for various various specimens as indicated respectively in curves l,

2 and 3 are 1/1 wavelength, 1 wavelength and 3,2 wavelength For the purpose of comparison, curve 4 represents the per cent transmission of light through 'an un- ;'coated plastic sheet of the same material and thickness..

Also, curve 5 is a plot of the per cent reflection of the l/z wavelength thickness coated cuprous iodide film on the 17g" thick Plexiglas" sheet. The other reflection curves tor the l wavelength and wavelength thick films .have not been plotted in -order to av'oid crowding of the graphical representations;

lt is to be noted, for example, that the vmasirnum transmission' of the wavelength thickness tilm coated on Plexiglas Vis approximately' 90% and that this maximum transmission occurs at a wavelength of about 575 millimicrons. This percentage transmission is only about 2/2% css than the transmission ci?- the unc'oated plastic sheet at the sarne wavelength of light. Similarly, it is seen that the maximum transmission of the 1 wavelengthoptical thickness tilru coated on an identical sheet of Plexigl as" occurs at approximately 530 millimicrons wavelength with a percentage transmission of.upp'roxi mately 86%'. Also, the wavelength thickness hlm indicates that n. maximum percentage transmission of 'approximately 84% occurs :it a wavelength of light of -5 55 millimicrons. Those latter per cent transmission' `values lalso represent an unusually. high transmission of" light as compared with the uncoated plastic at the 'same wavelength of light.

The reflection curve for the 1/2 wavelength film thickness further indicates that approximately only 7,5%

-light is reflected at that wavelength of light where maximum transmission occurs.

lf the absorption of light for the V; wavelength 'thick film is calculated over the entire visible spectrum, it will be found that its average value is about 295% for the entire range except in the region 400 to .430 rnillrncrons where' a very rapid increase occurs.

An'cxamination of the curves 'of Figure l shows the outstanding transmission characteristics, of these tilms. These high transmission values are also accompaniedby remarkably lownbsorption losses. The curves of Fig' ure l are repro'dt-xctions of those obtained from a recordving spectrophotometer.A

In my Work with the coated articles o f this invention, I have found that an electrical connection be 'made with the-'conducting films of cupr'ous iodide by several convenient means. Onel method is the application of colloidal ysilver paint to the hlm at a convenient point where electrical connection -is desired. Another method is the evaporation of gold to form a lplace of contact Where connection is desired. A third nieihodis pressure contact between a strip or ring of metal -such as brass, copper, silver or another conductor and the hlm. Each' of these 'methods provide for easy connection ofeie'ctrical,

conductors.

y l have found that protective tilms may he applied to the cupr'ous iodide coated articles of this invent-ion Where it is necessary or desirable to prevent mechanical action from injuring .the-se tilrns. Examples of 'agents -usable for `forming the protective medium are Krylonf' transparentV lacquers, and evaporated tilrns o f ysuitable linorganic salts,

"such as magnesium duoride and silicon monoxide. 'l'hese protective tilms do not appear to interfere with the conductivity of the cup'r-ous iodide tilrns.

.iilms on sheets of acrylic-plastic A having the dime small quantitybf iodine crystals -is placed in therbo'ttom of the jar which is then closed with a lid topreveat the es- Cape of iodine vapor; 'Die jar is then placed in an'oven and heated to that temperature at which it has beendete.:- mined by previous trials to be the best for production of tbe clearest tilrn of the desired thickness, The iodine vapor will be visible in the jar as a purpli'sh or dark 4brown gas. Upon reaching the proper ten'iperature the jar isV removed from the oven and the article to be-iodire'd isl quickly lowered into the'jar for a few seconds during which timejthecopper film is changed by the heated iodine vapor into an electrically conducting transparent {ilmof euprous iodide..

lFigure 3 diagra-mmatically illustrates a morerelined chamber which has been constructed for iodizingcopper sions of 7" x ll" x ls". This chamber consists of an inner iodiJing box 32 having'a topvor iid 34 and heating means lin the form o' a coiled insulated resistance wire 36 surrounding the outside. A small opening 33`rnay be provided in lid 34 for the purpose of receiving a string or wire 40 for suspending sheet A. In order to prevent the loss of heat as well as to distribute the heat more uniformly throughout the -iodi'zing box 32, a second box 42 is provided about the chamber 32 and the annular space annexes between the boxes 32'and lima-y be lled with a suitable insulating material 44 such as glass wool.

The operation ci'. the apparatus of Figure 3 is carried out by placing a few crystals 6 of iodine in the bottom of the chamber 32 and passing-a current through the resistance wire 36 of sullicient 'arnperage to produce a tcmperature of the optimum value which in the instance described is approximately 80 C. Upon attaining this tem- 'perature which may be measured by a thermometer inserted through the wall of chamber 32, the lid 3 4 isY removed and the plate to be coated is quickly lowered into the hot iodine vapor. immersion in the iodine vapor for a few seconds is sucient to convert the'thin copper film into a copper iodide. film which is transparent and electrically conducting. Upon removal from the iodiz-ing box 32 after completion of the reaction between the copper film and iodine, the surface may be colored dark brown because of excess iodine crystals which have condensed upon the surface, but these crystals evaporate quickly upon exposure of the plate Ato the air for a few moments.

in connection with my preferred method which has been described above along with the description of ap paratus suitable for carrying out 'the method, it is important to Vdetermine the quantity of copper which must be evaporated. in order to produce a film of a required thickness. Conversely, in order to prepare a cuprous Viodidefilm of specific thickness, a denite quantity of copper must be evaporated. AA convenient and siinple mcthodcf controlli-ng the quantity of copperto be evaporated is to use shortfmeas'ured lengths of standard copper wire of a convenient size. of 'wire using the same apparatus, conditions of tempera ture and pressue, arrangement of the'copper containing boats, and distance from these boats at which the article to be coated is placed, the same thickness o'f -liim 'may be obtained and it is `not necessary toweigh the copper .or perform other tedious operatic-ns in order toobtain a uniform lm each time articles are coated'.

The tinal films of cuprous iodide as has been explained-are exceedingly thin. The optical thickness should not exceed two wavelengths of light having a wavelength of By evaporating thesnme length' color may be used as a gage of lilrn thickness. In order 550 millirnicrons if it is desired to obtain atransparent lm. The amount of cop-per required to form such thin lms is exceedingly `small and consequently the amount of copper to be evaporated is also very small and it is dcsiroble, therefore, to have a convenient Ameans for accurately determining the required amount of copper.

No delinite ligure may be given to indicate the actual amount of copper which must be evaporated to produce a Vfilm of detnite lhcltnessbecause this ligure will vary .widely depending upon the geomeory and the operationof tht; particular coating .machine which is used as explained a ve.

evaporated 'from each individual boat of a number of boats will not bethe same as if the copper was to come from one boat or a smaller number of boats. Hence', itI follows that the quantity of copper to be evaporated in order to obtain a film of specific thickness must be determined for cach coating machine having different geometrical constructions andxcondit-iorss lof operation. .Having Aonce delcrmincd experimentally the amount to be evaporated for producinga' coating of a particular thickness, then it is possible to produce films of uniform thickness as long as the geometrical arrangement' of the coating machine and the conditions of operation are not disturbed.

ln my particular coating apparatus, two evaporating troughs or boats were used and they were spaced about 15" apart. Each boat was about 3" above the surface of the base plate. The article to be coated was placed on a horizontal support about 17" above the surface of the base plate and symmetrically located with respect to the two boats beneath it; Bare copper wire, size No. 24 B and S gage, was used at all times, it being apparent that other' sizes, of course,l could have been used.

Acuprous iodide tilm l/ wavelength thick has a reddishpurple color by reflection-as has been described, and this to obtain this particular thickness of film, an arbitrary length of No, 24 copper wire (for example l1/2") was placed in each boat and evaporated onto a piece Vof giass. Glass is convenient to use for these experiments since it is transparent, hard, and maybe used over and over again by cleaning olf the old films. The copper lilm was then iodized-in the manner described previously and the color of the reflected light was observed. lf 'the cole-r was not reddish-purple, the entire experiment would be repeated using slightly different lengths of wire. Proceeding in this manner I finally determined that for my particular equipment it wasnecessary to evaporate a DAG" length of No.

24v copper wire from each boat .in order to'V form a cup'rous iodide film having a reddish-purple color by reilecton.

Having determined 'the length of wire necessary to form a reddish-purple color by rciiection, it is necessary to perform one more experiment in. order to prove that producing a .'-j or 4,5 wavelength lilm. AIf thc 'resulting tilm i vis a brilliant yellowish-green, it indicatesthat the rst choice was probably a "l org wavelength li'trn. Further experiments would then' be necessary in order to determine the shortest' length of wire which Acould be evaporated to produce a reddish-'purple film. lfthe result ing film has practically no color upon using a piece of copper wire exactly lei :the llength o that determined for making the First reddish-purple color film by reflection theindcation is "that the length of lthe wire previously selectcd is correct for producing a ilm wavelength in thicknes.

It' is obvious 'that if the Aarticle being "coated is Ysupported ten inches from the evaporating boat in one Having thus determined the'properticngth of the wire to produce a film l/z wavelengthtb-ickit is easy to pro duce films of any other' thickness-by simply increasing or decreasing the length of the wire proportionately. Thus a tilm twice as thick can be made by cvaporating twice the normal lengthV of wire. A film l as thick may be made by evnporating the normal length of wire.

METHOD lI In carrying out Method II the cuprous iodidefilrn is formed in one-operation upon the surface of the article in a vacuum chamber such as that described in connection with Figure l of the `dra-r-Jings by evaporating pure uprous iodide powder. This method involves placing a quantity of cuprous iodide powder in the evaperating trough or .boat and evaporating directly in a high vacuum. Evaporation proceeds at a dull red heat. It is necessary to place a considerable quantity of cuprous iodide in the boat because at the evaporating temperature, decontposition of the iodide into free iodine and free'copper occurs rapidly. The iodine passes oi as vapor while the metallic copper remains in the boat. The cuprous iodide which does evaporate without decomposition -c'ondenses on the surface which it strikes to form an electrically conducting lm. v

Although the films produced by this method are identical in transparency and electrical conductivity with those produced by Method I, method Il possesses a number of Adisadvantages such a's the wastage of cuprous iodivo'ebecauseof the described decomposition. Further, iodine yapor contaminates thev pumping fluids in the vacuum pump and 4condenses in the cooler portion of the vacuum system. This contamination makes it dicult to obtain low pressures in the vacuum -chamberand necessitates frequent cleaning of the entire vacuum system. i

' METHOD lll I havealso discovered that transparentelectricaliy conducting lfilms of cuprous iodide may also be made hy iodizing ilnis of metallic copper which have been deposited from a chemical solution. Por the purpose of depositing coppi-:r tilrns on glass to malte copper mirrors a number or formulas for preparing chemical solutions are known and all of these formulas have in common the reduction' of a soluble saltof copper by a reducing agent incorporated in the solution. The free metal deposits as a lni on objects immersed in the solution. As an exam ple of one such solution which may be' tised is that.

resultant mixture should have-a clear yellow color. To

this solution was then added slowly and with constant stirring 87.5 cc. of a solution at C. of 11.1 grams ofpure potassium hydroxide in l0() cc. of distilled water. This solution should also be a clear yellow without any precipitate. i

The article to vbe Vcoated is placed .in 4this solutiorand the solution heated very gently, preferably in a water bath. The color of the solutic will gradually begin to 'change and the film of copperbegins to deposit on all of the surfaces of the article in' contact with the liquid.

The only way to determine the thickness of the 'copper film is to withdraw the article from the solution and make i() Although I have successilly made :films of. cuprous iodide which are transparent and electrically conducting by this method :it isthe least desirable of ythe three methods-because the thickness A:md-uniformity of the copper film is difficult to control. The rate of deposition of the 'copper upon thesurfacc ofthe article being coated is very sensitive to die cleanliness of the surface,

the temperature of the solution, and the agitation of the solution. f the surface. is not uniformly clean over its entire arca, then the copper deposits faster in some places than in .others and a -xion-t 1n iform coating results. The thickness of the Copper film is almost impossible to control because deposition .occurs at 'the same time that the-solution itself is changing color and becoming turbid and opaque as a result of the chemical reactions which are taking place in the liquid.

The above three methods for depositing a thin film of -cuprous iodide on a supporting surfacel are' the pre ferrcd methods; lflowever, other methods may be used for applying a 'thin copper tilm to a. surface lfor conversion to cuprous iodide. Among these methods are processes such as the spinne-ring" process in partial vacuum, the pyrolitic decomposition. of 'gaseous copper cornounds, and where applicable, electrolytc methods. Any P of these processes may be used where most convenient, and the scope of the invention is, therefore, no t'to be restricted by the illustrative methods of carrying out my process.

Each of the three methods Adescribed is not applicable indiscriminately to all substances which are to be coated. For example, either lyethod I or Method Il may be used for forming cuprous iodide vtilms on materials which are iiatensoluble, such as alkali halide crystals or those of the phosphate type, but therchemical method, Method ill, would obviouslybe unsuitable, since the crystals would dissolve in the aqueous copper depositing solution. Matcrials which are insoluble in water and are`not affected by the chemicals dissolved in the solution of Method-ill, may be tilmcd by the chemical method. Glass and ceramics,

' since they are insoluble in water and inert to n'iost chemicals, may be conveniently filmed by cllthree methods described.

The abovel process including :the three s eciically de scribed species and modifications thereof hen followed produces en electrically conducting coating or film on a surface of a supporting material 'without thencc-:ssity of using high temperatures for deposition such as those used by the prior nrt. Futherrnore,' the coatings or 4tilrns applied to the supporting surf-aces mayl be made transparent and transmit a very high percentage of visible light incident upon them.

The supporting surfaces coated with the films or' this invention in accordance-with the processof this intention possess a variety of uses'. For example, l have youtlined below speciticexarnples o? particular applications of the' coated materials lof thisitiiention. @ther uses than those outlined will, of course, .become apparent to those skilled in the art.

' Example I Accumulated staticcharges on various surfaces often offer disadvantages which are difficult to overcome. lf these surfaces, however, are coated by the cuprous iodide tilins of this invention it' ispossible to prevent the accumulation of static charges and overcome the diiculties encountered. may be 4conveniently coated with cuprous iodide, including heat sensitive surfaces, it is apparent that the Anur'ri'uer of applications for the concept.V of dissipatng electrical charges is many fold.

A specific example of a difficulty -otten rcvalertt is theaccumulation of a static electrical charge on the sight window of a sensitive electrical or mechanical meter which occasions interference with'the needle swing of the meter.

and thereby causes errors in .the reading of die same. Ey coating this sight ,window with a transparent electrically In view of the wide variety of surfaces which America, vol. 37 (October, i947), beginning on page 738.

Other specific uses of the coated articles of this inf venti'on will become apparentto those skilled in the art and I do not, therefore, wish to be limited to those disv10. A hea-t Vsensitive solidi-nett insulating body having on a surface thereof :Qn 'electrically conducting coating closed 'above which are for the purpose of illustration only.'

I claim:

1. A process for producing anelectrically conducting coating -on a surface ot" a solid inert insulating body whichv comprises thesteps of exposing said surface to an atmosphere of copper vapor and then exposing the resulting copper coated surface to iodine vapor.

2. A process forv producing an electrically conducting transparent coating on a surface of :1 solid inert insulating body which comprises the steps of exposing said surface to an atmosphere of copper vapor and then exposing the resulting copper coated surface to iodine vapor, the optical thickness o the resulting cuprou's iodide coating being from about 1A tovabout 2 wavelengths of monochromatic light' having a wavelength of 550 millimicrons.

3. A process for producing an electrically conducting coating on'the surface of a solid inert insulating body` which comprises the steps of exposing said surface under substantially vacuum conditions to an' atmosphere of copper vapor and then exposing the resulting copper coated surface to iodine vapor.

4. A process as set forth in c1aim'3 wherein said electrically conducting coating is transparent and has an optical thickness .of from about 1/4 to 2 wavelengths of monochromatic light having a wavelength of 550 millil microns. v

5. A process for producing an electrically conducting coating on a surface of a solid inert insulating body which comprises the steps of exposing said surface at substantially room temperature under substantially vacuurriv conditions to an atmosphere of copper vapor and then exposing the resulting copper coated surface which has a 'temperature of from about 20 C. to 23 C. to iodine vapor. i

6. A process as set^-forth in claim 5 wherein the temperature 'of said copper coated surface exposedftisiaide- F-jtj" iodine vapor is-cooled belowv about 20" C. to 23 C.

7. A process as, set forthin claim 6' wherein the ternperature of said iodine vapor is from about 2.0" C. to 100 C.

8. A solid inert insulating body having on asurface thereof an electrically conducting coating which cons-is essentially of cuprous iodide.- 9. A transparent solid inei't insulating 'Body having on a surface thereof an electrically conducting coating which4 consists essentially of cuprous iodide.

whichA consists essentiallyv oi cupi-ous iodide. lll. A solid inert insulating body having-on asu'rfacc thereof a transparent electric-:elly conducting coating which consists essentially of cuprous iodide.

12. A 'solid .inert insulatingv body hai/ ing ou atasurface thereof a transparent electrically conduet'ingcoat-ing of 'cuprous iodide havin-g. a thickness-of from about A to 2 wavelengthsof monochromatic light of aA wavelength of 550 millimicrons.

I3. An electromagnetic Awave energy shield .comprisa ing a solid inert vinsulating body having on a surface thereof an electrically conduetingicoati-ng which consists essentially of cuprous iodide, said coatingbeing grounded by the connection of a `conductor [from the coating to ground.

14. A condenser coinprising a dielectric material hav ing coated .on opposite surfaces thereof electrodes of cuprous iodide.

l5. A light valve comprising a. transparent electroopt'ic crystal having opposed surfaces thereof coated with transparent electrodes of cuprous iodide.

1 6. An electromagnetic wave energy shieldas in claim 13 wherein bothl the said insulating body and the said surface thereof which comprises a-'transparent inert .im sulating body having a .grounded electrically conductingtransparent coatiu g of-cuprou-s iodide o'n 'a surface thereof.

References Cited inthe tile of this patent UNITED STATES PATENTS 1,694,276 Mohrle Dec. 4, 1928 1,947,112 'Ruben Feb. 13, 1934 1,964,322 Hyde June '26, 1931i` 2,002,221 Van Geel et al. May 21, 1935 2,026,086 Farncornb Dec.` 31,1935 2.955,5@ Okoli'c'sanyi Sept. 29, 1936 Littleton May 2,4, 1938 2,175,016 Brunke Oct. 3, 1939 YV2,239,452 Williams et al. Apr. 22, 19,41 2,352,085 Dinamick June 20, 1944 2,366,687. Osterber-g Ian. 2, 1945 2,428,600' Williams Get. 7, 1947 2,429,420- McMaster.. ;Oct. 2 1, 1947 2,467,325 Mason Agp'r. 1 2, 1949 2,493,200 Land Ian.A 3, 1950 2,498,003 Paterson Fcb.'21, 1950 2,577,936 Waggoner Dec. 11, 1951 

1. A PROCESS FOR PRODUCING AN ELECTRICALLY CONDUCTING COATING ON A SURFACE OF A SOLID INERT INSULATING BODY WHICH COMPRISES THE STEPS OF EXPOSING SAID SURFACE TO AN ATMOSPHERE OF COPPER VAPOR AND THEN EXPOSING THE RESULTING COPPER COATED SURFACE TO IODINE VAPOR. 